Method for producing pearlitic malleable iron



Jan. 23, 1968 R. c. DAVIS ET AL 3,365,335

METHOD FOR PRODUCING PEARLITIC MALLEABLE IRON 5 Sheets-Sheetl Filed D80. 4, 1964 INVENTORS RALPH C. DAVIS 8 BRUCE R. SHUE @M ww ATTORNEYS Jan. 23, 1968 c, DAV|$ ET AL 3,365,335

METHOD FOR PRODUCING PEARLITIC MALLEABLE IRON Filed Dec. 1, 1964 5 Sheets-Sheet 2 INVENTORS RALPH C. DAVIS a BRUCER.SHUE

ATTOR N EYS Jan. 23, 1968 R. c. DAVIS ET AL 3,365,335

METHOD FOR PRODUCING PEARLITIC MALLEABLE IRON 5 Sheets-Sheet 3 Filed Dec. 4, 1964 NINVENTORS RALPH C. DAVIS & BY BRUCE R. SHUE W M? ATTORNEYS 8y O0: 009 0C2 v oE mm G &w 6. mm 5 G g G aw 5 m 6 Rm F H r w m WE & QR Av NW & g & w. w w *w v vw m Mm vw Mw ncdofiwnwwo n0 k g w Q 000 000000 000 A w A Q A g w E S/Gwm mm m6 5% mw W w G 8 8 a 8 8 5 r 1000 N mZON QUZON D mZON Q MZON MUZON N UZQN UZON WAVE Patented Jan. 23, 1968 3,365,335 METHOD FOR PRODUCING PEARLITIC MALLEABLE IRON Ralph C. Davis, Ironton, and Bruce R. Shue, Dayton,

Ohio, assignors to The Dayton Malleable Iron Company, Dayton, Ohio, a corporation of Ohio Filed Dec. 4, 1964, Ser. No. 416,086 5 Claims. (Cl. 148139) This invention relates to pearlitic malleable iron castings, and particularly to an improved method for annealing as cast white iron to form pearlitic malleable iron having improved physical properties.

Pearlitic malleable has become an increasingly popular material for high-impact, high-strength parts such as universal yokes, automotive differential gear cases, automotive automatic transmission parts, truck gear support cases, crankshafts, disc brake caliper castings, truck brake spiders and the like. In physical appearance, pearlitic malleable looks like any other ferrous casting but differs significantly because of the versatility of physical properties which can be provided. For example, yield strength may be varied from 45,000 to 100,000 p.s.i., tensile strength up to 125,000 p.s.i., modulus of from 25 million to 27 million p.s.i., Brinell hardness may vary from 160 to 500. In modulus of elasticity, pearlitic malleable compares quite favorably with austenitic stainless steel which has a modulus of about 28 million p.s.i. Pearlitic malleable on the other hand offers the advantage of good castability because of the relatively high carbon content, i.e., melt carbon content of around 2.55%, and yet the product exhibits ductility, high aggerga-te strength and is relatively easily machined because much of the carbon in the final product is in nodular form. When compared to standard ferritic malleable iron, pearlitic exhibits a wider range of properties and can be quench hardened to three times the yield strength of standard malleable.

Pearlitic which is an iron silicon carbon alloy containing around 1.45% silicon (melt content) in addition to the percentage of carbon previously noted is usually formed by a direct process in which scrap, foundry returns are the raw materials. Melting is carried out in a cupola, air or induction furnace followed by refining during which metallurgical inspection is closely controlled to provide the proper chemical composition of the white iron. Following the refining operation, the white iron castings are poured and heat treated under conditions which are controlled in such a manner that a portion of the carbon is converted to nodules while the remainder is retained in combined form. The combined carbon is in the form of spheroids, pearlite lamellae, or tempered martensite.

As cast white iron of malleable composition will solidify with the carbon which is present in the material being in the form of cementite or iron carbide, and when at room temperature, will consist of rather large carbides and pearlite, that is, alternate layers of ferrite and cementite. The malleabilization procedure converts the combined carbon into elemental carbon, that is, graphite, and ferrite. The formation of pearlitic malleable includes a first-stage rnalleabilization or graphitiza-tion procedure in which the castings are heated through the eutectoid range to transform the pearlitic into austenite in which carbon from the cementite diffuses into the iron to form a solid solution of carbon in gamma iron. The first-stage graphitization includes several processes which are car ried out simultaneously including solution of the cementite at its interface with austenite, dissolution of disassoci-ated cementite into iron and carbon, ,migration of carbon through the austentite or diffusion of matrix atoms away from the nuclei from which the temper carbon grows, and

precipitation of graphite. After first-stage graphitization, the structure of the casting consists of graphite, also referred to as temper carbon nodules, which are distributed through the austenite matrix, the latter being a solid solution of gamma iron saturated with an amount of carbon which is dependent upon the particular temperature of the first-stage malleabilization procedure. Usually the firststage malleabilization is carried out at a temperature of between 1700 and 1800 F.

The objective in the formation of pearlitic malleable is to treat the product of the first-stage graphitization in such a manner that the eutectoidal carbides and low-temperature transformation products are purposely retained. Various procedures have been used commercially in the formation of pearlitic malleable iron including first-stage graphitization, air quench, and temper; or followed by the additional steps of reheating into the austenitic range, oil quench and temper, or ferritic malleabilization (ferrite and free carbon) followed by reheating into the austenitic range, oil quench and temper. The matrix may vary, for example, it may be lamellar pearlite, spheroidite, martensite, tempered martensite, fine spheroidite, coarse lamellar pearlite or bainite.

The process of forming pearlitic malleable in accordance with the present invention produces a matrix which is highly spheroidized tempered martensite, and somewhat less acicular in form than some of the pearlitic malleable iron heretofore made commercially.

Accordingly, it is a primary object of the present invention to provide an improved process for the manufacture of pearlitic malleable iron castings.

Another object of the present invention is the provision of a pearlitic malleable iron having a highly spheroidized tempered martensitic structure and relatively low degree of surface decarburization.

Another object of the present invention is an improved process for the manufacture of pearlitic malleable iron including an arrest annealing operation which provides control of the combined carbon and wherein the material is oil quenched.

A further object of the present invention is an improved process for the continuous formation of pearlitic malleable iron castings of improved physical characteristics while at the same time reducing the time and cost involved in pearlitic malleabilization.

A further object of the present invention is the provision of improved apparatus for pearlitic malleabilization of ferritic castings.

Other objects and advantages of the present invention will be apparent from the following description, the accompanying drawings and the appended claims.

In the drawings FIG. 1 is a diagrammatic view in perspective of a pearlitic malleable annealing oven constructed in accordance with the present invention;

FIG. 2 is a diagrammatic view partly in section and partly in elevation of the discharge and quenching apparatus in accordance with the present invention;

FIG. 3 is a diagrammatic side view of the oven in accordance with the present invention showing the position of the burners, sight glasses, and thermocouples;

FIG. 4 is a time and temperature chart illustrating the sequence of heating and cooling cycles in the formation of pearlitic malleable iron in accordance with the present invention; and

FIGS. 5, 6, and 7 are graphs showing the milling and turning speeds of pearlitic malleable iron in accordance with the present invention as compared to the milling and turning speeds of conventionally processed pearlitic malleable iron.

Referring to FIG. 1, which illustrates a preferred embodiment of the apparatus of the present invention, an oven 11 is provided with a work tray entrance 12 and pusher cylinders 13 which operate to advance the work trays through the various heating zones of the oven. As shown, the oven includes seven zones to 26 so that the castings introduced into the oven are processed from firststage graphitization all the way to the pearlitic malleabilization. During passage through the oven, the atmosphere of the oven is controlled by the use of inert gas, for example, nitrogen circulated through the oven at a rate of about 3200 cubic feet per hour, thereby substantially eliminating the presence of any gaseous materials which tend to bring about scaling and decarburization of the castings. From zone 26, the castings are advanced from a work tray exit 28 to a tray dump mechanism 29 to an oil quenching station generally designated 30. The furnace is a radiant tube controlled atmosphere pusher type furnace with various zones, the first zone 20 being a heating zone, zone 21 being a heating and holding zone, zone 22 being a fast cooling zone, 23 being a reheat zone, 24 being a heating zone, and zones 25 and 26 being holding zones.

The quenching station includes a quench basket dumping and Yo-Yo mechanism 32 and a quench oil tank .33 and work basket 34. Referring to FIG. 2 which shows more of the details of the discharge from the oven and the oil quench system, the castings from the oven 11 are advanced in a work basket 35 to a basket rocker 36 which dumps the castings into the quench oil tank 33 positioned vertically below the level of the oven exit. The castings from the oven are dumped into the oil in about 10 to 30 seconds after they leave the oven. The oil in the quench tank is constantly circulated and maintained under controlled temperature of between about 130 to 200 F. and preferably in the range of 130 and 140 F., the tank being provided with an agitator oil inlet 37 and an agitator oil outlet 38. A recirculation outlet 41 is provided for withdrawing oil and cooling it and returning it to the quench oil tank 33. Provision is also made for a drain 42 located at the base of the tank. The level of the oil is shown at 43 slightly below the overflow passage 45.

The tank is capable of holding approximately 4,000 gallons of quench oil while the work baskets are capable of holding between 500 and 800 pounds of castings thereby providing a relatively high ratio of oil to weight of castings. After the oil quench which takes approximately eight to ten minutes, the quench basket 34 is raised by the dumping mechanism 32 and the quenched castings are dumped into a collection tote box 50. The quenching basket dumping mechanism is supported vertically above the quench oil tank by a plurality of elevator guide frames and equipment support columns 54. The quench oil may be Sohio Quench Oil No. 74, which has an acid number of 1.12 (calorimetric) a Saybolt Universal viscosity at 100 F. and 210 F. of 76.8 seconds and 37.2 seconds respectively. It is preferred that sufficient oil be used to provide an oil to castings ratio of not less than about 3.5 to 4 gallons of oil per pound of castings. Other quench oils may also be used, for example, Sinclair 521.

The advantages of the apparatus above described are that the castings are in a controlled atmosphere during the malleabilization annealing and reheating cycles and the oven is capable of controlled heating and cooling through the various zones. In this way, the castings are virtually scale free while the controlled heating and cooling operates to provide pearlitic malleable castings of improved properties as will be described herein below.

Further, the oven is designed for continuous operation, with the pusher cylinders advancing the castings through the various zones and the oil quench operation taking place immediately as the castings leave the exit side of the oven. The entire apparatus is relatively compact considering the nature of the operations involved and thus, the space required is reduced. Another advantage of the apparatus of this arrangement is the vertical lifting of the 4 quenched castings which reduces the cost of the quenching operation by reducing drag-out of removal of the quenching oil during removal of the quenching oil during removal of the castings. By raising the quenched castings vertically out of the bath, the oil which is dragged-out is allowed to drain back into the bath.

Referring to FIG. 3, the details of the oven are shown indicating the position of burners 61 which are on one side of the oven and a second set of burners 62 which are on the other side of the oven. The oven also includes thermocouples 63 of three groups, one group to control the burners directly, a second group of thermocouples which operate to check the thermocouples controlling the burners, and a third set of thermocouples which operate as safety elements to prevent overheating of the oven. Thus, the burners are direct thermocouple controlled to provide wide variations in the temperature from one zone to the next.

Each of the zones of the oven also includes a sight glass 64 permitting visual observation of the castings during the passage through the oven. Additionally, zone 4 includes a multiplicity of cooling tubes 65 which permit a reduction of the temperature of zone 4 during the cooling operation as will be described more fully hereinbelow.

The cycle for the formation of pearlitic malleable iron in accordance with the present invention is an arrest annealing operation followed by an oil quenching operation. A preferred time and temperature cycle is shown in FIG. 4 wherein the abscissa represents time, with each graduation representing between 30 and 40 minutes while the ordinate represents temperature ranging from 0 to 1800 F. FIG. 4 has been arranged with respect to FIG. 3 to show the correlation of oven stations to temperature of the various stations of the oven. The total time of the castings in the oven is between about 12 to 18 hours during which the castings are taken through the first-stage graphitization, a low temperature annealing cycle and a reheating cycle prior to the oil quench treatment. The dwell time of the castings in each zone of the oven is controlled by the frequency of pushes of cylinders 13. In zones 1 and 2 of the oven, the temperature of the castings is raised above the eutectoid or critical range to bring about graphitization of the relatively large carbides. The temperature for first-stage graphitization may be between 1700 and 1800 F., and the castings are held at this temperature for a sufiicient period of time to alter the structure of the white iron thereby providing a matrix of austenite through which temper carbon nodules are disturbed. After first-stage graphitization, the castings consist of a solid solution of gamma iron and carbon.

In accordance with the present invention the castings are brought to a temperature of between 1700 F. and 1800 F. in about 5 to 6 hours and preferably held at a temperature of 1750" for between approximately 2 to 4 hours in zones 2 and 3, followed by a reduction in temperature in one to three hours to between about 1200 and 1650 F. This reduction in temperature operates to control the combined carbon while controlling the structure of the iron to eliminate quench cracking which tends to occur when the castings are oil quenched directly after first-stage graphitization. As shown in FIG. 4, the temperature of the castings is reduced over a period of approximately one to two hours to 1400 F., and if the temperature is to be reduced to 1200 F., it requires approximately another one-half to one hour. The castings may be held at this cool temperature for a period of one-half to three hours. Following the reduction in temperature, the castings are raised to a temperature above the cooling temperature and in the range of between 1550 to 1650" F. for a period of time varying between one to two hours. The reduction in temperature occurs primarily in zone 4 and the temperature is raised in zone 5 and held in zones 6 and 7, and thereafter the casting im- 7 mediately dumped into the quench oil in about ten seconds.

It is also possible in accordance with the present invention to utilize a cycle in which the temperature of the castings is raised for approximately 1300' to 1675 F. at the rate of 200 F. per hour. Thereafter the temperature is increased to 1750 F. over a period of seven hours and held at that temperature for approximately four hours. Thereafter, the temperature of the castings are reduced at the rate of 130 F. per hour to a temperature of about 1200 F. and held at that temperature for approximately an hour followed by raising the temperature of the castings at 200 F. per hour to approximately 1650 F. followed by the oil quenching operation.

The advantages of the cycles previously described over some of the procedures heretofore employed is the fact that it is a continuous cycle in a controlled atmosphere requiring only one quenching operation While at the same time so conditioning the castings as to reduce the susceptibility to quench cracking while at the same time providing a structure which is highly spheroidic tempered martensite. For example, pearlitic malleable iron may be made by a procedure which includes first-stage graphitization followed by an air quench operation and thereafter reheating the castings to 1600 F. in a direct fired oven followed by an oil quench. While castings formed by this particular procedure exhibit acceptable qualities, the improved procedure of the present invention offers substantial advantages in that the scale on the castings is substantially eliminated, the castings exhibit uniform hardness, the tendency for the castings to crack during oil quenching is markedly reduced, while improving the overall physical characteristics of the pearlitic malleable iron by controlling the combined carbon in the castings. The improved physical properties include lower impact transition temperatures, higher impact properties, the castings respond uniformly to local hardening and are more easily machined.

Following the quenching operation, the castings processed in accordance with the present invention are drawn or tempered to the specified hardness to provide a pearlitic malleable iron having a highly spheroidized tempered martensitic structure. This may be done by heating the castings to between 12-00 to 1340 F. for a specified period to the hardness desired.

Photomicrographic analysis of test bars of pearlitic malleable made in accordance wit-h the present invention of ASTM grade 60003 processed as follows:

F. First-stage graphitization 1750 Cooled 1500 Reheat and discharge temperature 1575 Oil temperature 140 Hardness after oil quench 555 BHN (Brinell hardness number).

Draw temperature 1220 and having a cehmical analysis of:

Si 1.51 Mn .42 C 2.59 S .139 Cr .037 P .051

exhibited a highly spheroidized tempered martensite structure and showing the following physical characteristics:

Ultimate strength 95900 Yield 75700 Elongation in two inches, percent 7.5 BHN 207 Additionally, photomicrographs at a magnification of 100 times indicated less than .002 inch decarburization, i.e., loss of surface carbon, while the normal range is from .010 to .015 inch. Other samples of pearlitic made in accordance with the present invention showed less than .008 inch decarburization. Photomicrographic examination at 800 times magnification indicated a highly spheroidic tempered martensite structure which was very fine grained thereby providing better machinability.

In comparison, pearlitic mellable formed by first-stage graphitization, air quenching, reheating to 1600 F. in a direct fired oven followed by an oil quenching wherein the temperatures of the castings prior to oil quenching was 1640 F., the oil temperature F., the hardness of the casting after quenching 550 BHN, and subsequently drawn or tempered at 1220" F., showed a more acicular tempered martensite structure, that is, as opposed to the fine grained tempered martensite of the pearlitic malleable made in accordance with the present invention. The chemical analysis of the comparison standard was as follows:

Si 1.51 Mn .42 C 2.59 S .139 Cr .037 P .051

The physical analysis provided the following data:

Ultimate strength 99500 Yield 75050 Elongation in two inches, percent 7.0 BHN 217 This material was also an ASTM grade 60003.

Comparison of these results indicates that even though the pearlitic malleable of the present invention has a lower hardness than the comparison product, the product of the present invention has a higher elongation and yield strength. As a general rule, as the hardness increases, ulti mate and yield increase while elongation decreases. Thus, it follows that as hardness increases to 217 BHN, the ultimate and yield will be noticeably higher than that of the comparison standard.

A comparison of ASTM grade 60003 pearlitic malleable for machinability demonstrates the improved characteristics of the product of the present invention. A first group of test bars was prepared by a process which included arrest annealing, air quenching, reheating, oil quenching and draw or temper to specification period. A second group of test bars was prepared in accordance with the process of the present invention and comparative tests were undertaken for face milling both the skin and under the skin. The test conditions and results are shown in FIG. 5 which indicates that pearlitic malleable formed in accordance with the present invention could be machined at 20% higher milling speeds than the test bars of the first group. Additionally, the tool life at any given milling or tuning speed is longer when machining pearlitic malleable wherein castings were made in accordance with the present invention due to the presence of fine spheroids in the matrix of martensite.

In a series of turning tests comparing samples of grade ASTM 60003 (Group I) prepared in accordance with the present invention and a second set of samples of the same ASTM grade (Group II), the results of which are shown in FIGS. 6 and 7, the pearlitic malleable of the present invention showed 86% higher cutting speeds with a tool life of thirty minutes in skin cuts and 47% higher cutting speeds at a thirty minute tool life in under the skin cuts.

The turning tests were conducted on a 16 inch x 30 inch American Pacemaker Lathe equipped with a 30 HP. variable speed drive. The test specimens were hollow cylinders having an CD. of 5 inches and an ID. of 3 inches and were 12% inches long. The test cylinders were held in a three jaw chuck and supported by a large live center at the tail stock end. Cutting speeds were varied from to 1150 feet per minute, and in the skin tests the feed rate was .030 inch/revolution, and .015 inch/revolution for the underskin turning tests. In the skin tests, the depth of cut was .100 inch and the depth of cut from under-skin tests was .062 inch.

7 The tools were Kennametal grade K-6 (C-2) carbide tools used with standard negative rake angle tool holders with the following geometry:

Back rake, degrees 1 5 Side rake, degrees 1 5 Side cutting edge angle, degrees End cutting edge angle, degrees 15 Relief, degrees 5 Nose radius, inches .030

1 Negative.

The cutting fluid was a water-soluble oil, and all tests were discontinued at a wearland of .015 inch or actual tool failure, whichever occurred first.

In addition to the above tests, turning tests with ASTM grade 80002 made in accordance with the present invention exhibited 36% faster speed for a tool life of 30 minutes than 80002 pearlitic malleable used in The Malleable Founders Society Program. In under-skin turned cuts, the material of the present invention was 22% faster at a tool life of thirty minutes than the comparison standard.

The continuous process of the present invention by which white iron castings of malleable composition are heat treated in a controlled atmosphere to form pearlite malleable by an arrest annealing and oil quenching operation represents an improved procedure in that only a single quenching operation is used. For example, the prior art methods include at least two separate heating cycles While the present invention utilizes only one continuous heating cycle, followed in each case by a tempering operation. The susceptibility of the castings of the present invention to crack due to the quenching operation is substantially eliminated by the cooling and reheating cycle, during one pass through the oven which follows first-stage graphitization. Additionally, the continuous method of the process simplifies, to a great extent, the mechanical handling of castings which are to be processed to form pearlitic malleable iron, while providing castings which are easier to machine and exhibit better physical properties than those formed by some of the prior methods.

While the methods, products and apparatus of the present invention illustrate preferred embodiments of the invention, it is to be understood that the invention is not limited to these precise forms of methods, products and apparatus, and that changes may be made therein without departing from the scope of the invention which is defined in the appended claims.

What is claimed is:

1. The method of preventing quench cracking in the formation of pearlitic malleable iron from white iron castings of malleable composition which include massive carbides, wherein said white iron castings are converted to pearlitic malleable iron of highly spheroidized tempered martensite by annealing said castings followed by oil quenching and tempering, said method comprising the steps of continuously advancing said castings through an oven having an inert atmosphere therein, said castings being heated in said oven to a first temperature in the range of 1700 F. and 1800" F. for a period of time sufiicient to efifect graphitization of said carbides, cooling said castings in said oven from said first temperature to a second temperature between 1200 F. and 1600 F. and maintaining said castings exposed to said temperature for a period of time from one-half to three hours for controlling the combined carbon, thereafter heating said castings in said oven to a third temperature higher than said second temperature and in the range of 1400 F. to 1650 F. for a period of from one-half to three hours, rapidly introducing said castings into quench oil at a temperature of between about F. and 200 F., and thereafter tempering said castings to a predetermined desired hardness.

2. A method as set forth in claim 1 wherein said castings are heated in said oven at a rate such that the castings are at said first temperature in not less than about five hours, and wherein said castings are cooled in said oven from said first temperature to said second temperature in from one to three hours.

3. A method as set forth in claim 2 wherein said castings are held at said second temperature for a period of between about one-half and one and one-half hours.

4. A method as set forth in claim 1 wherein said castings are heated to a temperature of about 1650 F. in a first zone, advancing said castings to a second zone and heating them to a temperature of about 1750 F., holding said castings at 1750 F. for a period of between one and three hours, advancing said castings to another zone and cooling them to said second temperature in one to three hours, holding said castings at said second temperature for a period of one to one and one-half hours, and thereafter advancing said castings to a third heating zone for raising the temperature thereof to said third temperature in one-half to one and one-half hours, said third temperature being in the range of 1550 F. to 1650 F.

5. The method as set forth in claim 1 wherein the ratio of quench oil to castings during said quenching operation is at least about 3.5 gallons of oil per pound of castings.

OTHER REFERENCES Foundry Trade Journal, Aug. 20, 1936, relied on pp. -138.

CHARLES N. LOVELL, Primary Examiner.

DAVID L. RECK, Examiner. 

1. THE METHOD OF PREVENTING QUENCH CRACKING IN THE FORMATION OF PEARLITIC MALLEABLE IRON FROM WHITE IRON CASTING OF MALLIABLE COMPOSITION WHICH INCLUDE MASSIVE CARBIDES, WHEREIN SAID WHITE IRON CASTING ARE CONVERTED TO PEARLITIC MALLEABLE IRON OF HIGHLY SPEHEROIDIZED TEMPERED MARTENSILE BY ANNEALING SAID CASTINGS FOLLOWED BY OIL QUENCHING AND TEMPERING, SAID METHOD COMPRISING THE STEPS OF CONTINUOUSLY ADVANCING SAID CASTING THROUGH AN OVEN HAVING AN INERT ATMOSPHERE THEREIN, SAID CASTINGS BEING HEATED IN SAID OVEN TO A FIRST TEMPERATURE IN THE RANGE OF 1700*F. AND 1800*F. FOR A PERIOD OF TIME SUFFICIENT TO EFFECT GRAPHITIZATION OF SAID CARBIDES, COOLING SAID CASTINGS IN SAID OVEN FROM SAID FIRST TEMPERATURE TO A SECOND 